Publications by Year: 2011

Stergiopoulos T, Kontos AG, Likodimos V, Perganti D, Falaras P. Solvent effects at the photoelectrode/electrolyte interface of a DSC: A combined spectroscopic and photoelectrochemical study. Journal of Physical Chemistry C [Internet]. 2011;115:10236-10244. WebsiteAbstract
The behavior of organic solvents typically used in redox electrolyte media for dye-sensitized solar cells (DSCs) has been systematically investigated by resonance Raman spectroscopy in combination with electrochemical impedance (EIS), intensity modulated photovoltage (IMVS), and photocurrent (IMPS) spectroscopies. Resonance Raman spectra reveal appreciable shifts in the vibrational frequency of the dye carboxyl anchoring groups as well as intensity variations of high- and low-frequency modes of dye-redox species by varying the electrolyte solvent and the polarization bias. These results are related to the variable surface coverage of the dye-TiO2 photoelectrode by solvent molecules determined by their donor number, the concomitant change on the concentration of dye-redox couple intermediate adducts, and the dye-TiO 2 electronic coupling. EIS and IMVS reveal a marked variation of the recombination kinetics and chemical capacitance of the corresponding DSCs, providing direct evidence for the shift of the TiO2 conduction band edge and deceleration of back-reaction kinetics. This accounts well for the solvent dependence of the device opencircuit voltage, whereas more complicated effects underlie the variation of the short-circuit current density that depends on electron injection. Tuning the solvents' function in DSCs may thus result in significant improvement of the DSC photovoltaic efficiency by effectively controlling electron energetics and injected photoelectron-triiodide recombination. © 2011 American Chemical Society.
Romanos GE, Likodimos V, Marques RRN, Steriotis TA, Papageorgiou SK, Faria JL, Figueiredo JL, Silva AMT, Falaras P. Controlling and quantifying oxygen functionalities on hydrothermally and thermally treated single-wall carbon nanotubes. Journal of Physical Chemistry C [Internet]. 2011;115:8534-8546. WebsiteAbstract
The effects of hydrothermal and thermal treatments on surface oxygen functionalization of single-wall carbon nanotubes (SWNTs) were quantitatively investigated by means of water adsorption/desorption, temperature-programmed desorption (TPD), Raman spectroscopy, thermogravimetric analysis, and nitrogen porosimetry. SWNTs hydrothermally treated under mild acidic conditions were compared to highly purified reference materials heavily functionalized under aggressive reflux conditions. Water adsorption/desorption and TPD analysis were successfully combined to determine the nature, concentration, thermal stability, and acidic strength of the oxygen functional groups on the SWNT's surface. These results were correlated to Raman spectroscopy data that allowed identifying the marked evolution of the defect-activated phonon modes of SWNTs. The concomitant charge transfer effects were differentiated through the distinct variation of both first- and second-order Raman modes as a function of the amount and acidity of the surface oxygen groups as well as the SWNT's chirality. In addition, analytical investigations on thermally treated SWNTs in mild oxidative (in air) and pyrolytic conditions (under Ar) confirmed the formation of amorphous carbon that depends primarily on the acidification process, although a significant fraction of functional groups remains attached on the SWNTs' walls rather than on carboxylated carbonaceous fragments. Quenched solid density functional theory (QSDFT) analysis of the bimodal pore size distribution of the functionalized SWNTs revealed pronounced variations of the underlying microporous and mesoporous structure, associated with the diverse effects of the packing between SWNT bundles and the closer aggregation of individual carbon nanotubes upon surface oxidation and thermal treatment. The SWNTs functionalization procedure can be effectively controlled and quantified, and the optimum conditions can be defined in relation to the desired physicochemical properties and pore structure characteristics for specific applications. © 2011 American Chemical Society.
Kontos AG, Pelaez M, Likodimos V, Vaenas N, Dionysiou DD, Falaras P. Visible light induced wetting of nanostructured N-F co-doped titania films. Photochemical and Photobiological Sciences [Internet]. 2011;10:350-354. WebsiteAbstract
Nitrogen and fluorine co-doped TiO2 films have been prepared by dip coating of a modified titania sol-gel based on a nitrogen precursor and a nonionic fluorosurfactant as pore template and fluorine source. The modified NF-TiO2 films absorb in the visible spectral range, between 400-510 nm and undergo reversible hydrophilic conversion under visible light to a final contact angle of 8°, in contrast to the UV limited optical response of their undoped anatase TiO2 analogues. The phenomenon takes place at a rate slower than the corresponding one observed for the UV stimulated superhydrophilic effect. The wetting response of the N-F doped TiO2 films correlates well with the variation of their optical properties and surface morphological characteristics and most importantly with their photocatalytic activity, rendering these materials very promising for self-cleaning applications under visible light. © 2011 The Royal Society of Chemistry and Owner Societies.
Kontos AG, Likodimos V, Vassalou E, Kapogianni I, Raptis YS, Raptis C, Falaras P. Nanostructured titania films sensitized by quantum dot chalcogenides. Nanoscale Research Letters [Internet]. 2011;6. WebsiteAbstract
The optical and structural properties of cadmium and lead sulfide nanocrystals deposited on mesoporous TiO2 substrates via the successive ionic layer adsorption and reaction method were comparatively investigated by reflectance, transmittance, micro-Raman and photoluminescence measurements. Enhanced interfacial electron transfer is evidenced upon direct growth of both CdS and PbS on TiO2 through the marked quenching of their excitonic emission. The optical absorbance of CdS/TiO2 can be tuned over a narrow spectral range. On the other side PbS/TiO2 exhibits a remarkable band gap tunability extending from the visible to the near infrared range, due to the distinct quantum size effects of PbS quantum dots. However, PbS/TiO2 suffers from severe degradation upon air exposure. Degradation effects are much less pronounced for CdS/TiO2 that is appreciably more stable, though it degrades readily upon visible light illumination. © 2011 Kontos et al.
Han C, Pelaez M, Likodimos V, Kontos AG, Falaras P, O'Shea K, Dionysiou DD. Innovative visible light-activated sulfur doped TiO2 films for water treatment. Applied Catalysis B: Environmental [Internet]. 2011;107:77-87. WebsiteAbstract
Visible light-activated sulfur doped TiO2 nanocrystalline films were synthesized by a sol-gel method based on the self-assembly technique with nonionic surfactant to control nanostructure and an inorganic sulfur source (i.e., H2SO4). The films were characterized by UV-vis diffuse reflectance, XRD, TEM, Raman, AFM, ESEM, XPS, FT-IR, EDX, EPR and porosimetry. The results showed that the physicochemical properties of the films, such as BET surface area, porosity, crystallite size and pore size distribution could be controlled by the calcination temperature. The highest surface area, smallest crystallite size and narrow pore size distribution were obtained for sulfur doped TiO2 films calcined at 350°C, which exhibit very smooth surface with minimal roughness (<1nm). The optical absorption edge of sulfur doped TiO2 was red shifted with indirect bandgap energy of 2.94eV. Sulfur species distributed uniformly throughout the films were identified both as S2- ions related to anionic substitutional doping of TiO2 as well as S6+/S4+ cations, attributed mainly to the presence of surface sulfate groups. EPR measurements revealed a sharp signal at g=2.004, whose intensity correlated with the sulfur content and most importantly was markedly enhanced under visible light irradiation, implying the formation of localized energy states in the TiO2 band gap due to anion doping and/or oxygen vacancies. In terms of photocatalytic activity, films calcined at 350°C were the most effective for the degradation of hepatotoxin microcystin-LR (MC-LR) under visible light irradiation, while films calcined at 400°C and 500°C degraded MC-LR to a lower extent, following the evolution of the sulfur content with calcination temperature. The photocatalytic activity of the sulfur doped TiO2 film was stable during three consecutive experiments under visible light irradiation, confirming the mechanical stability and reusability of the doped nanostructured thin film photocatalysts. © 2011 Elsevier B.V.
Chen Y, Lunsford SK, Song Y, Ju H, Falaras P, Likodimos V, Kontos AG, Dionysiou DD. Synthesis, characterization and electrochemical properties of mesoporous zirconia nanomaterials prepared by self-assembling sol-gel method with Tween 20 as a template. Chemical Engineering Journal [Internet]. 2011;170:518-524. WebsiteAbstract
Zirconium dioxide nanomaterials, including nanoparticles and their derived films, were synthesized using a new surfactant self-assembling sol-gel route, which involves complexation of zirconium alkoxide precursor with acetylacetone as a chelating agent and Tween 20 as a structural directing agent. The properties of the synthesized material were studied using XRD, SEM, SAXS, HRTEM and N2 adsorption and desorption analysis. It was found that Tween 20 can greatly increase the BET surface area and pore volume, inhibit crack formation in the derived film and retard particle growth at some extent. There exists tetragonal crystalline network with narrow pore size distribution in the ZrO2 nano-materials prepared with Tween 20 in the sol. Furthermore, a mechanism on the formation of mesopore ZrO2 structure via the Tween 20 self-assembling sol-gel process is presented. With the selection of the sono-gel carbon electrode modified by the mesoporous ZrO2 film (M-ZrO2 film) prepared with 1.10g Tween 20/ml zirconia sol, enhanced electrochemical responses (i.e., enhanced peak currents) to catechol and ascorbic acid were confirmed using cyclic voltammetry (CV). Moreover, the modified electrode demonstrated additional improved electrochemical properties, such as good reversibility (ipc/ipa≈1) and high selectivity (exceptionally resolved oxidation peaks between catechol and ascorbic acid), good stability and repeatability. © 2010 Elsevier B.V.